Natural Rocky Shores Research Articles

Intertidal assemblages on groynes along sandy shores in Portugal: Exploring the effects of orientation and distance to rocky shore

Coastal erosion is becoming increasingly problematic as sea level rise and coastal areas become more urbanised. In response, more defence structures such as groynes are being built, which are crucial for counteracting sediment loss and coastline retreat. Despite worldwide use, comparatively little is known about the factors that determine the species composition on such structures. In this study, the composition and abundance of intertidal benthic species on groynes was investigated as a function of groyne orientation (North vs. South) and distance to natural rocky shores (5 km vs. 42 km). While orientation showed no effect on benthic assemblages, distance to rocky shores was identified as a key factor influencing the assemblage composition. Macroalgae were found in greater abundance further away from rocky shores, while snail and barnacle species were found in greater numbers closer to rocky shores.

Investigating the interactive effects of habitat type and light intensity on rocky shores

Light availability and habitat complexity are two key drivers of community assembly. Urbanisation has been shown to affect both, with important consequences to ecological communities. On the intertidal, for instance, studies have shown that light intensity is greater on natural rocky shores than on less complex artificial habitats (seawalls), though different habitats can also experience similar light intensities, for example when shaded by urban structures. Understanding therefore how these factors individually, and combined, affect communities is important to understand the mechanisms driving changes in community structure, and consequently provide solutions to tackle the increasing homogenisation of habitats and lightscapes in urbanised spaces through smart infrastructure designs. Here, we assessed how different light levels affect the recruitment of communities in rock pools and on emergent rock on an intertidal rocky shore. We cleared 30 patches of emergent rock and 30 rock pools and manipulated light using shades with different light transmissions (full light, procedural control, 75%, 35%, and 15% light transmission, full shade) and assessed mobile and sessile communities monthly for 6 months. Effects of reducing light levels were generally stronger on rock than in pools. Fully shaded plots supported double the amount of mobile organisms than plots in full sunlight, in both habitats. Algal cover was higher in pools compared to rock, and at intermediate light levels, but effects varied with site. This study highlights the importance of variable light conditions and different habitats for rocky shore communities, which should be considered in future coastal developments to retain natural biodiversity.

Artificial rockpools create habitat refugia on seawalls at high tide

Rockpools are fundamental habitats on natural rocky shores that provide refugia for marine life at low and high tide. Yet, artificial coastal structures lack the topographical complexity seen in these natural habitats. Eco-engineering, that may include the deployment of artificial rockpools, attempts to address the lack of suitable habitat on coastal infrastructure but most studies focus on species abundance metrics at low tide. It is important to understand how eco-engineering interventions may provide habitat at high tide compared to the surrounding artificial substrate.In this study, we demonstrate how groups of rockpools (1, 3 or 5 rockpools) add habitat complexity at high tide to a concrete seawall in Poole Harbour, UK. Between April and October 2022, eighteen GoPro cameras were deployed to record species richness and abundance in artificial rockpools and the adjacent concrete sea wall. Additionally, the length of time the most abundant fauna (the shanny fish Lipophrys pholis and the European shore crab Carcinus maenas) spent engaging in different behaviour (resting, feeding, moving) was recorded.Overall, there was no significant difference in the abundance and species richness of mobile fauna using the artificial rockpools versus the seawall. However, both the shanny and shore crab generally spent more time in the rockpools than on the seawall. Both crabs and shanny preferentially engaged in feeding behaviours on the seawall, which has been attributed to the high percentage cover of barnacle prey. Crabs and shanny spent more time resting in the rockpools than the seawall and the only reproductive behaviour observed occurred within the rockpools. Our work suggests that artificial rockpools support the habitat needs and multiple life history requirements of these species at high tide. Both the seawall and rockpools provide valuable resources, which further emphasises the need for variety in eco-engineering feature designs.

Metal bioaccumulation in marine invertebrates and risk assessment in sediments from South African coastal harbours and natural rocky shores

Industrial and urban activities are major contributors to metal contamination in coastal systems, often impacting the physiology, distribution and diversity of marine invertebrates. This study assessed metal contaminations in sediments, seawater, algae and invertebrates across four armoured systems (harbours) and two natural sites along the south coast of South Africa. Bioaccumulation factors such as Biosediment (BSAF), Biowater (BWAF), Bioaccumulation (BAF) and bioremediation of metals by invertebrate bioindicators were also determined. Spatial variation in metal concentrations were observed, however, bioaccumulation of metals was site and species-specific. Invertebrates bioaccumulated higher metal concentrations in armoured than natural sites, with filter feeders exhibiting higher concentrations than grazers. Among filter feeders, Octomeris angulosa and Crassostrea gigas bioaccumulated elevated aluminium (Al), arsenic (As), chromium (Cr), zinc (Zn) and copper (Cu), while, Perna perna accumulated elevated nickel (Ni), cadmium (Cd) and lead (Pb). Among grazers, Siphonaria serrata and Scutellastra longicosta bioaccumulated elevated Al, Cr, Cd, cobalt (Co), Cu, Ni and Zn. Bioaccumulation factors indicated that (As, Ni, Zn) were bioaccumulated by algae, and invertebrates from sediment (BSAF>1) and from seawater (BWAF>1). Additionally, invertebrates bioaccumulated metals from their prey item, algae as indicated by (BAF>1). Arsenic Cd and Pb in invertebrates were above the maximum limit set for human consumption by various regulatory bodies. Our findings underscore the significant role of coastal invertebrates in bioaccumulating and bioremediating metals, suggesting a natural mechanism for water quality enhancement, especially in urbanised coastal areas.

Relative predation intensity of an intertidal gastropod on artificial coastal defense structures.

Despite seawalls becoming ubiquitous coastal features, and having some physical similarities to natural rocky shores, it remains unclear how these urban habitats influence predator-prey interactions. Predators can affect intertidal mobile prey densities through two pathways: (1) successful predation directly influences prey mortality rates, and (2) direct and indirect effects of predation can scare and induce motile prey to seek safer areas. In this study, we investigated whether intertidal predators affect the density of the marine gastropod, Nerita undata, at four seawall sites in Singapore. Using a tethering method that we developed, we monitored the mortality and other evidence of predation (shell state) of tethered N. undata. Field experiments revealed high (22.5%-82.5%) predation potential of N. undata across the four sites, with significantly higher predation risk at lower shore heights and for snails with mixed shell coloration. Observations and analysis of the shell state after 3 days showed that predation on seawalls was primarily by crushing predators such as fish. Other predators of N. undata include predatory snails, with various feeding methods that left behind different predator signatures. Our results add substantially to the limited knowledge on predator-prey interactions on seawalls, particularly for Nerita undata, and suggest that seawall systems are more dynamic than previously thought. This further highlights the role of these artificial structures as important habitats and feeding grounds in urban coastal ecosystems.

Species contributions to ecosystem stability change with disturbance type

Simultaneous exposure to multiple stressors complicates the challenge of predicting ecological responses to global environmental change. Here, we show that the contributions of individual species and functional groups to the overall stability of ecosystems can be modified by the presence of different stressors, both individually and in combination. By disturbing natural rocky shore communities with nutrients and sediments and simulating extinction of predatory whelks and grazers, we also found that consumers can simultaneously stabilise and destabilise communities along different stability dimensions, irrespective of their trophic position. Our results suggest that our experimental disturbances influenced consumer contributions to stability indirectly by modifying the interactions between consumers and macroalgae in different ways. These findings merit further exploration in different systems exposed to a range of different stressors to better understand how perturbations of different kinds can modify the multifaceted contributions of species to the overall stability of ecosystems.

Population size and movement ecology of intertidal gastropods on rocky shores and seawalls in Singapore

ABSTRACTIncreasing coastal development and global warming have resulted in large-scale habitat changes, with artificial coastal structures replacing extensive tracts of natural shores. In Singapore, for example, more than 63% of the natural coastline has been replaced by seawalls. Multiple studies from both temperate and tropical regions have compared species diversity supported by these artificial structures with natural rocky shores. Few, however, have estimated and compared the population size and movement of common intertidal species between these two habitat types. Using mark–recapture techniques, this study investigated: (1) the population size of three common gastropod genera (Nerita spp., Trochus spp. and Turbo spp.) and (2) differences in displacement of Nerita spp. and Trochus spp., two common species found on natural rocky shores and seawalls in Singapore. The results of our mark–recapture surveys indicated that seawalls supported large densities of Nerita spp.—more than 50 times greater than that on adjacent rocky shores. The mark–recapture data also revealed that movement of the gastropod species differed between the two habitats, with individuals on seawalls generally travelling longer distances.

Population structure and reproductive states of the dogwhelk Nucella lapillus differ between artificial structures and natural rocky shores

Artificial structures are an increasingly common feature of coastal marine environments. These structures are poor surrogates of natural rocky shores, and generally support less diverse communities and reduced population sizes. Little is known about sub-lethal effects of such structures in terms of demographic properties and reproductive potential, both of which may influence the dynamics and long-term viability of populations. This study examines the population structure, reproductive states and embryo production of Nucella lapillus populations on artificial structures and natural shores in Ireland and Wales. Population density was measured twice at six natural shores and six artificial structures: once in winter and once in spring. At each sampling, the shell height of 100 individuals from each site was measured. Monthly collections of adult specimens and egg capsules were made at each site from November–January and from March–May, in order to determine sex ratios, reproductive states, and embryo abundances. Artificial structures supported larger individuals and very few juveniles compared to natural shores. Between December and January, natural shores experienced a distinctive pulse in spawning activity followed by a decline in the proportion of females in a reproductive state, whereas on artificial structures the proportion of reproductive females remained relatively stable. Differences observed may be due to a lack of microhabitats on artificial structures, along with subtle variations in structure slope. Eco-engineering interventions, including the addition of refugia such as cracks and crevices, may allow N. lapillus populations on artificial structures to approximate those on natural shores.

Artificial rockpools: Seaweed colonisation and productivity vary between sites but are consistent across environmental contexts

Artificial structures are widespread features of coastal environments, but are poor surrogates of natural rocky shores because they generally support depauperate assemblages with reduced population sizes. This has generated significant interest in eco-engineering solutions, including retrofitting seawalls with artificial rockpools to increase water retention and provide microhabitats. Although these have proven effective at individual sites, widespread uptake is contingent on evidence of consistent benefits across a range of contexts. In this study, Vertipools™ were retrofitted on eight seawalls in different environmental contexts (urban v rural and estuarine v marine) along the Irish Sea coastline and were monitored regularly for two years. Seaweed colonisation proceeded in a manner similar to patterns described for natural and artificial intertidal systems in general, consisting of early dominance by ephemeral species followed by the appearance and eventual establishment of perennial habitat-formers. After 24 months, species richness did not differ between contexts, but differed between sites. The units supported populations of large habitat-forming seaweeds at all sites. Productivity and community respiration of the colonising communities differed between sites by up to 0.5 mg O2 L−1 min−1, but not across environmental contexts. This study demonstrates that bolt-on rockpools attract similar levels of biotic colonisation and functioning in a variety of temperate environmental contexts, and could be considered for widespread implementation as an eco-engineering solution.

Colonisation after disturbance on artificial structures: The influence of timing and grazing

Artificial structures are poor surrogates of natural rocky shores, meaning they generally support depauperate assemblages. These differences may result from a combination of recruitment processes, biotic interactions, and structuring by environmental factors. In this study, plots were cleared on two seawalls and two natural shores at two separate timepoints – in August 2020 (summer) and February 2021 (winter) - and monitored over one year to determine the influence of timing of disturbance on recruitment and succession. Additional plots were cleared at one of the seawalls at a single timepoint in August 2020, and exclusion cages were installed to determine the influence of grazing pressure on colonisation; these were monitored for 18 months. Disturbance during winter resulted in higher concentrations of all biofilm components up to 3 months, but did not impact benthic community composition beyond this point. Grazer exclusion on artificial structures increased biofilm concentrations and influenced community composition in comparison to plots on artificial structures without exclusion, while communities on natural surfaces differed in terms of species composition to those on artificial plots at 12 months. We conclude that the timing of routine maintenance works on artificial structures may impact initial biofilm abundances. Furthermore, while grazing pressure does influence community structure on artificial structures, this alone is not sufficient to explain biological differences between artificial structures and natural shores.

The BioGeo Ecotile: Improving biodiversity on coastal defences using a multiscale, multispecies eco-engineering design

Hard coastal defences support lower biodiversity than natural rocky shores. Ecological enhancement on coastal structures can improve biodiversity by increasing habitat heterogeneity. Most studies have investigated the effect of only one type of texture on intertidal biodiversity. There is a lack of eco-engineering designs that mimic the complexity of natural rocky shores and are scalable for real world applications and commercial manufacturing. To address these gaps, we developed a novel, multiscale (mm-cm), multispecies design called BioGeo Ecotile that is scalable and readily manufacturable. The hybrid design combines previously tested eco-engineering features (pits, holes, grooves and crevices), providing habitats for a range of intertidal organisms. To test the success of the design, Ecotiles and smooth tiles were deployed on rock armour and flood walls along Edinburgh's coast, Scotland. Post-deployment, data on species presence and abundance were collected at the start and end of the second settlement season. Textured Ecotiles supported higher species richness (F3,55 = 21.18, p < 0.001) and colonisation than smooth tiles and adjacent rock armour. Ecotiles supported more mobile species, some of which (crabs) were not recorded on the other treatments. Material type (concrete vs rock) significantly affected community composition, where concrete was dominated by fucoids and rock by barnacles. In this temperate setting, the Ecotiles have enhanced biodiversity of rock armour achieving practical conservation goals. This is the first known retrofit of tiles onto rock armour in the UK. The tiles can be scaled up to whole walls or rock armour units. We demonstrate that a science-design approach can achieve ecological and engineering goals simultaneously, which can accelerate widespread implementation of eco-engineering in large-scale projects.

The population structure, sex ratio and reproductive potential of limpets (Patella spp.) on natural shores and artificial structures in the Irish Sea

Artificial structures often support depauperate communities compared to natural rocky shores. Understanding variation in ecological success across shore types, particularly regarding habitat-forming species or those with structuring roles, is important to determine how artificial structure proliferation may influence ecosystem functioning and services. We investigated the population structure, sex ratio and reproductive potential of limpets on natural shores and artificial structures on Irish Sea coasts. Limpets were generally less abundant and Patella vulgata populations were often male dominated on artificial structures compared to natural shores, suggesting that shore type may influence these factors. P. vulgata length varied across sites within the Irish Sea (nested in coast and shore type) in autumn/winter, as well as temporally across sites along the Welsh coast. There was no difference in the proportion of P. vulgata in advanced stages of gonad development across shore types. The results suggest that rip-rap artificial structures may provide a habitat comparable to natural shores, however, the addition of ecological engineering interventions on artificial structures may allow limpet populations to better approximate those on natural shores.

Multiple genetic sources facilitate the northward range expansion of an intertidal oyster along China's coast.

Coastal artificial structures on the former mudflats provide available habitats for the rocky intertidal species which can establish new populations in these emerging habitats over their former distribution range limits. As a former southern species, the oyster Crassostrea sikamea has become a pioneer and rapidly invaded the artificial shorelines in northern China. We used a seascape genomics approach to investigate the population structure and genetic sources of C. sikamea on the coastal artificial structures, which is crucial for understanding the genetic mechanisms driving species distribution range expansion and invasion pathway of intertidal species. Five C. sikamea populations, including two artificial substrate populations (WGZ and ZAP), one oyster reef population (LS), and two natural rocky shore populations (ZS and XM), were measured using single nucleotide polymorphism (SNPs) obtained from double digest restriction-site associated DNA sequencing (ddRAD-Seq). Redundancy analyses (RDA) were implemented for investigating the relationship between local temperature variables and the temperature adaptability of C. sikamea. Genetic diversity, direction and strength of gene flow, and population structure all revealed that the LS and ZS populations were the genetic sources for the oyster populations on the emerging northern coastal artificial structures. Results of RDA showed that there were different adaptive potentials for northern and southern populations to local temperature variables and the oyster reef population which frequently suffers from heat stress owned high heat adaptability. The ZS population as a genetic source nearby the Yangtze River estuary provided mass larvae for the northern populations, and the other genetic source, the heat-tolerant LS population, in the oyster reef played an important role in the post-settlement success by providing preadapted genotypes. These results highlight the importance of multiple sources with divergent adaptative capabilities for biological invasion, and also emphasize the importance of the oyster reef in coastal biodiversity and conservation.

Hermit crabs use vision, olfaction, and eavesdropping to detect potentially available gastropod shells on a natural rocky shore

Intertidal marine crustaceans use multimodal cues to find resources, relying most heavily on chemical cues. However, water movement on intertidal shores acts as a sensory stressor, diluting and modifying the direction of waterborne chemicals, which may increase the relative importance of visual cues. We compared the influence of visual cues, chemical cues, and their combination in attracting free-living hermit crabs (Calcinus californiensis) to potentially available shells in shallow natural wave-protected tide pools. We used the tissue of dead gastropods as a chemical cue and clean, varnished shells as a visual cue from the gastropod species whose shell is most preferred by C. californiensis. The visual, chemical, or combined (bimodal) cue was placed in the center of a circular arena. We counted the crabs that entered the arena and classified their behavior into one of three mutually exclusive categories: touching the stimulus, remaining in the arena without reaching the stimulus, or leaving the arena without touching the stimulus. We ranked the quality of each hermit crab's current shell and recorded the time they took to reach the stimulus. Since more than one hermit crab participated in each trial in the field, we explored potential eavesdropping by comparing the time required to reach the stimulus among the first three hermit crabs. Cue type did not affect the number of crabs that entered the arena. However, most hermit crabs that entered the arena reached the chemical and bimodal shell cues; in contrast, with the visual cue alone most crabs remained in the arena without touching the stimulus, and those that touched it took longer to do so. Hermit crabs were more likely to touch the stimuli when a bimodal cue was presented in the arena and when they were in a broken shell (a low-quality refuge). Interestingly, there was only an apparent eavesdropping effect in the unimodal visual cue; subsequent crabs touched the visual stimulus more quickly once a pioneer crab had already touched it. Previous demonstrations that C. californiensis can visually discriminate shells suggest that the reduced number of crabs that touched the visual stimulus and their longer latency to do so was not due to low visual acuity or inability to interpret the image. Rather, it is likely due to the incongruous set of cues (visible shell without odor), which may reduce the perceived value or certainty about the shell, therefore the benefit/risk relationship in the natural environment. Our results highlight the importance of matching chemical sensing with vision as a trigger for decision-making in dynamic and unpredictable environments.

Weak effects of age but important role of microhabitats in community differences between breakwaters and natural rocky shores across a latitudinal gradient

AbstractAimCoastal infrastructures are increasing at different latitudes, and their deployment often results in a complete replacement of natural habitats. Although infrastructure provides novel habitats, ecological engineering can increase their similarity to natural rocky habitats. One hurdle for effective design of greener infrastructure is the scarce information on the processes that underpin differences in the structure of communities between built and natural habitats. Using long‐term data of intertidal community structure, we tested whether the magnitude of between‐habitat differences changes with breakwater age and across latitudes, and we examined the role of topographic microhabitats (i.e., rock pools) in driving these differences.LocationNine locations distributed from 18 to 41°S.Time period2016–2020.MethodsWe sampled the mid‐intertidal community structure on breakwaters of different age and on natural rocky habitats (platforms and boulder fields) and assessed the availability of microhabitats, such as crevices and rock pools, along a coastline spanning 23° of latitude. Using generalized additive models (GAMs), we assessed the effect of the different factors on species diversity and functional group abundances.ResultsAge since construction was a poor predictor of differences between breakwaters and natural habitats. Communities on older breakwaters bore a poor resemblance to the species composition or functional group abundances found in natural habitats. The magnitude of differences between breakwaters and natural habitats varied with latitude, with season and with the type of natural habitat attributable to differences in microhabitats, especially in rock pool availability.ConclusionsOur results indicate that replacement of rocky habitats with breakwaters will cause permanent alterations to coastal ecosystems. Our findings suggest that the availability of microhabitats able to shelter species from stressful environmental conditions is more important at lower and intermediate latitudes; hence, they should be incorporated primarily into infrastructures in tropical or subtropical areas. These strategies could help to compensate for the impacts on rocky ecosystems associated with increases in coastal urbanization.

The ecological benefits of adding topographic complexity to seawalls vary across estuarine gradients

Artificial structures such as seawalls increasingly dominate marine urban environments. As compared to natural rocky shore habitats, seawalls are usually flat, featureless, vertical surfaces that support reduced biodiversity. One approach to increase their biodiversity is to add topographic complexity (‘complexity’) that increases microhabitat diversity and surface area. Initial investigations of the effects of complexity on the biodiversity of marine built structures found positive relationships, but more recently spatially variable effects have been found at a biogeographical scale. The present study tested whether at the scale of sites within an estuary (in this case Sydney Harbour), effects of complexity also spatially vary, and whether pollution and estuarine gradients are good predictors of this variation. Comparisons of intertidal communities colonising flat and complex (creviced / ridged) tiles, affixed to seawalls confirmed that effects of complexity were spatially variable at the site-scale, ranging from neutral to highly positive. Proximity to stormdrains, a point source of contaminants including metals, was a poor predictor of complexity effects, but effects were generally greater in the outer harbour, where biodiversity was overall greater, than the inner harbour. These results suggest that eco-engineering interventions based on complexity will not have universally positive effects and instead vary between sites separated by as little as hundreds of meters. Knowledge of spatial variation in physico-chemical conditions and the size and composition of the species pool of available colonists may assist in predicting when and where adding complexity to marine built structures will be of ecological benefit.

Estuarine Infauna Within Incidentally Retained Sediment in Artificial Rockpools

Artificial coastal structures (ACSs) are primarily designed to provide services for human use, such as flood defence or shipping, and are generally poor for marine biodiversity. Consequently, there has been significant research effort to enhance these hard structures to increase biodiversity and habitat availability via eco-engineering. On seawalls and breakwaters, this has included the creation of habitats for benthic species found on natural rocky shores, including the provision of cracks, crevices and water retaining features, such as artificial rockpools. When sediment retention in these features has occurred, it has often been deemed detrimental to the overarching aim of the intervention. Yet, it is soft sediment habitat that is impacted the most through coastal construction. As ecological enhancement of a flood defence scheme, nine concrete retrofit rockpools were installed at three different tidal elevations between mean high water neap tide and mean tide level on steel sheet piling on the Arun Estuary in Littlehampton Harbour, United Kingdom, which naturally filled with mud 1 year after installation. To explore how analogous the faunal assemblages and sediment profile of rockpool mud were to two local mudflats, core samples were taken and analysed for species richness, abundance, biomass, assemblage structure, median grain size, and organic matter content. More benthic species were observed in the artificial rockpool than in the local mudflats. Although the rockpools were placed at higher tidal levels than the lower shore mudflat, their assemblage structure and species richness were more similar to the lower shore mudflat at the base of the sheet piling than the upper shore mudflat. This study demonstrates that retained sediment within eco-engineered features on hard ACSs can create habitat for benthic assemblages. Providing sediment-retentive features on ACSs has the potential to provide a novel eco-engineering option that may be appropriate for some heavily modified waterbodies on sheltered, depositional coasts.

Influence of coastal engineering on the intertidal macrobenthic community in the Dongtou Islands, China

Abstract Coastal areas have been intensively urbanized in Asia, causing changes to the structure and function of intertidal macrobenthic communities. In this study, the intertidal assemblages on two natural rocky shores, two rocky shores disturbed by construction works, and two shores with mixed artificial seawalls and mudflats in the Dongtou Islands were studied. We examined the influence of coastal engineering by comparing macrobenthic community structures in different transects. The statistical results revealed significant differences between macrobenthic community structures in natural and influenced transects, although all the high tidal zones shared common species. The most common species on mudflats were opportunistic polychaetes; however, historical evidence suggests that the mudflats in Dongtou used to be dominated by Mollusca. We inferred that this change in community structure occurred when the seawalls were constructed five5 years previously. The macrobenthic community does not appear to have subsequently recovered. We also observed an absence of habitat-forming macroalgae in our study area, although similar investigation results revealed that they still prevail on an uninhabited island in Dongtou. The increasing disturbance caused by construction work may explain the decline in brown algae, which are mainly distributed in the low tidal zone.

Artificial shorelines lack natural structural complexity across scales.

From microbes to humans, habitat structural complexity plays a direct role in the provision of physical living space, and increased complexity supports higher biodiversity and ecosystem functioning across biomes. Coastal development and the construction of artificial shorelines are altering natural landscapes as humans seek socio-economic benefits and protection from coastal storms, flooding and erosion. In this study, we evaluate how much structural complexity is missing on artificial coastal structures compared to natural rocky shorelines, across a range of spatial scales from 1 mm to 10 s of m, using three remote sensing platforms (handheld camera, terrestrial laser scanner and uncrewed aerial vehicles). Natural shorelines were typically more structurally complex than artificial ones and offered greater variation between locations. However, our results varied depending on the type of artificial structure and the scale at which complexity was measured. Seawalls were deficient at all scales (approx. 20–40% less complex than natural shores), whereas rock armour was deficient at the smallest and largest scales (approx. 20–50%). Our findings reinforce concerns that hardening shorelines with artificial structures simplifies coastlines at organism-relevant scales. Furthermore, we offer much-needed insight into how structures might be modified to more closely capture the complexity of natural rocky shores that support biodiversity.

Fucus vesiculosus populations on artificial structures have potentially reduced fecundity and are dislodged at greater rates than on natural shores

Artificial structures are widespread features of coastal marine environments. These structures, however, are poor surrogates of natural rocky shores, meaning they generally support depauperate assemblages with reduced population sizes. Little is known about sub-lethal effects of such structures, for example, in terms of demographic properties and reproductive potential that may affect the dynamics and long-term viability of populations. Such understanding is particularly important for ecosystem engineer species, such as the intertidal seaweed Fucus vesiculosus. In this study, F. vesiculosus was sampled on eight artificial structures and eight natural shores along the east coast of Ireland and the west coast of Wales. Algal percentage cover, biomass, density of individuals, and growth rate did not differ between artificial and natural shores. Growth and reproductive cycles were consistent with previous studies for this species. While there was considerable variation from site to site, on average, populations on natural shores produced a higher number of mature receptacles during the peak reproductive period in April, and lower rates of dislodgement than on artificial structures. As F. vesiculosus reach peak reproductive output after 24 months, this suggests that individuals may be removed from populations on artificial structures before reaching their full reproductive potential. In this case, this did not influence density, percentage cover, or biomass, which suggests that F. vesiculosus populations on artificial structures may function similarly to those on natural shores if supported by suitable source populations, but potentially may not persist otherwise.